Secondary objectives are: to determine the severity and frequency (measured by flu-like symptom score [FLS-S]) of FLS in these participants; to determine the duration (measured in number of hours) of FLS in these participants; to determine the effect of BIIB017 on other participant-reported outcomes, including treatment satisfaction (measured with the Treatment Satisfaction Questionnaire for Medication [TSQM]) and disability status (measured with the Patient Determined Disease Steps [PDDS]) over a 56-week period; to determine whether interferon-related FLS result in missed days of work/daily activities (e.g., absenteeism); to assess the use of additional medications (in addition to current medications used to treat FLS) to relieve BIIB017-related FLS; to determine the incidence of adverse events throughout the study period; to characterize the immunogenicity profiles of participants switching from prior IFN-β therapy to BIIB017.